Typically you need to be in the industry or at least have a business license to buy wholesale. Often retail nurseries will cut you a good deal if you buy a single plant type in quantity - I'd check with them and see. Otherwise, you might see if there is a landscaper you know that would have wholesale access and can help you out.

Check out Classy Groundcovers.
They're not wholesale, but they specialize in selling large quantities of groundcover plugs.
I've had wonderful experience with them.
I've bought dwarf mondo grass and liriope which were so full that their pots were nearly bursting.
And plants arrive usually the week after ordering, which is remarkable.
Check out their website.

@marrilyn--You may want to check with neighbors or on craigslist. Folks with a lot of groundcover are usually very willing to give away whatever you need if you provide the labor to remove it (in fact, you're welcome to all the English Ivy from my back yard that you want).

I've also had luck just driving around on Saturdays and finding garden items that had been "cleaned out" and left for pick-up at the street (always ask first before loading it up). This includes plant materials, shrubs, and hardscaping.